Optical waveguide circuits having laterally tilted waveguide cores

ABSTRACT

A photonic integrated circuit (PIC) in which some optical waveguides have laterally tilted waveguide cores used to implement passive polarization-handling circuit elements, e.g., suitable for processing polarization-division-multiplexed optical communication signals. Different sections of such waveguide cores may have continuously varying or fixed lateral tilt angles. Different polarization-handling circuit elements can be realized, e.g., using different combinations of end-connected untilted and laterally tilted waveguide-core sections. In some embodiments, laterally tilted waveguide cores may incorporate multiple-quantum-well structures and be used to implement active circuit elements. At least some embodiments beneficially lend themselves to highly reproducible fabrication processes, which can advantageously be used to achieve a relatively high yield of the corresponding PICs during manufacture.

BACKGROUND Field

Various example embodiments relate to optical communication equipmentand, more specifically but not exclusively, to photonic integratedcircuits (PICs).

Description of the Related Art

This section introduces aspects that may help facilitate a betterunderstanding of the disclosure. Accordingly, the statements of thissection are to be read in this light and are not to be understood asadmissions about what is in the prior art or what is not in the priorart.

PICs are widely used, e.g., in various elements and components offiber-optic networks. Some PICs are implemented using material platformsin which the refractive indices of the optical-waveguide core andcladding differ significantly. For such PICs, the transverse electric(TE) and transverse magnetic (TM) polarizations can have a relativelylarge difference in their group indices, i.e., effective refractiveindices. For example, TE and TM polarization modes in representativesilicon-based, on-chip waveguides have group indices of about 4.1 and2.8, respectively. Group-index differences of such magnitude candisadvantageously make it relatively difficult to construct and/orfabricate a functional PIC capable of appropriately handlingpolarization-division-multiplexed (PDM) optical communication signals.

SUMMARY OF SOME SPECIFIC EMBODIMENTS

Disclosed herein are various embodiments of a PIC in which some opticalwaveguides have laterally tilted waveguide cores used to implementpassive polarization-handling circuit elements, e.g., suitable forprocessing PDM communication signals. Different sections of suchwaveguide cores may have continuously varying or fixed lateral tiltangles. Different polarization-handling circuit elements can berealized, e.g., using different combinations of end-connected untiltedand laterally tilted waveguide-core sections. In some embodiments,laterally tilted waveguide cores may incorporate multiple-quantum-well(MQW) structures and be used to implement active circuit elements.

At least some embodiments beneficially lend themselves towell-controllable and highly reproducible fabrication processes, whichcan advantageously be used to achieve a relatively high yield of thecorresponding PICs during manufacture.

According to an example embodiment, provided is an apparatus, comprisinga photonic integrated circuit that comprises: a substrate having asubstantially planar main surface; and an optical waveguide attached tothe substrate adjacent and along said main surface, the opticalwaveguide including a waveguide core; and wherein a first section of thewaveguide core has a first edge whereat first and second surfaces of thewaveguide core are connected to one another, the first surface beingsubstantially orthogonal to said main surface, the second surface havinga nonzero lateral tilt angle with respect to said main surface.

In some embodiments of the above apparatus, the nonzero lateral tiltangle changes continuously along a longitudinal waveguide direction ofthe first section between a first nonzero tilt-angle value and adifferent second nonzero tilt-angle value.

In some embodiments of any of the above apparatus, the nonzero lateraltilt angle is greater than 5 degrees but smaller than 60 degrees.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge whereat the first surface and athird surface of the waveguide core are connected to one another, thethird surface having the nonzero lateral tilt angle with respect to saidmain surface.

In some embodiments of any of the above apparatus, the second edge iscloser to said main surface than the first edge.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge whereat the first surface and athird surface of the waveguide core are connected to one another, thethird surface having a smaller nonzero lateral tilt angle with respectto said main surface than the second surface.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge whereat the first surface and athird surface of the waveguide core are connected to one another, thethird surface being parallel to said main surface.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core comprises a semiconductor diode.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core comprises a multiple-quantum-well structure having aplurality of semiconductor layers, at least some of said semiconductorlayers being parallel to the second surface.

In some embodiments of any of the above apparatus, a second section ofthe waveguide core is untilted with respect to said main surface, thefirst and second sections of the waveguide core being end-connected toone another.

In some embodiments of any of the above apparatus, the photonicintegrated circuit comprises a polarization splitter-rotator thatincludes the first and second sections of the waveguide core.

In some embodiments of any of the above apparatus, a second section ofthe waveguide core has a different nonzero lateral tilt angle, the firstand second sections of the waveguide core being end-connected to oneanother.

In some embodiments of any of the above apparatus, a third section ofthe waveguide core is untilted with respect to said main surface, thesecond and third sections of the waveguide core being end-connected toone another.

In some embodiments of any of the above apparatus, a third section ofthe waveguide core is untilted with respect to said main surface, thefirst and third sections of the waveguide core being end-connected toone another.

In some embodiments of any of the above apparatus, the photonicintegrated circuit comprises a polarization splitter-rotator thatincludes the first, second, and third sections of the waveguide core.

In some embodiments of any of the above apparatus, the optical waveguidecomprises a ridge-waveguide structure attached to the substrate adjacentand along said main surface, the ridge-waveguide structure having firstand second sidewalls substantially orthogonal to said main surface, thefirst sidewall including the first surface.

In some embodiments of any of the above apparatus, the ridge-waveguidestructure includes at least a portion of a waveguide cladding, saidportion being farther from said main surface than the first section ofthe waveguide core.

BRIEF DESCRIPTION OF THE DRAWINGS

Other aspects, features, and benefits of various disclosed embodimentswill become more fully apparent, by way of example, from the followingdetailed description and the accompanying drawings, in which:

FIGS. 1A-1C show schematic views of different parts of an opticalwaveguide circuit according to an embodiment;

FIGS. 2A-2D show schematic cross-sectional views of ridge-waveguidestructures according to several alternative embodiments of the opticalwaveguide circuit shown in FIGS. 1A-1B;

FIG. 3A pictorially illustrates an example manufacturing process thatcan be used to make the optical waveguide circuit of FIGS. 1A-1Caccording to an embodiment;

FIGS. 3B-3C schematically show in-plane layouts of a dielectric maskthat can be used in some embodiments of the example manufacturingprocess corresponding to FIG. 3A;

FIG. 4 shows a schematic cross-sectional view of a ridge-waveguidestructure according to yet another embodiment of the optical waveguidecircuit shown in FIGS. 1A-1B; and

FIGS. 5A-5D show schematic views of an optical waveguide circuitaccording to another embodiment.

DETAILED DESCRIPTION

As used herein, the term “photonic integrated circuit” (or PIC) shouldbe construed to cover planar lightwave circuits (PLCs), integratedoptoelectronic devices, wafer-scale products on substrates, individualphotonic chips and dies, and hybrid devices. Example material systemsthat can be used for manufacturing various PICs may include but are notlimited to III-V semiconductor materials, silicon photonics (SiP),silica-on-silicon products, silicon-on-insulator (SOI) products,silica-glass-based PLCs, polymer integration platforms, Lithium Niobateand derivatives, nonlinear optical materials, etc. Both packaged devices(e.g., wired-up and/or encapsulated chips) and unpackaged devices (e.g.,dies) can be referred to as PICs.

PICs can be used for various applications in the telecommunications,instrumentation, and signal-processing fields. Many of the existing PICapplications are in the sub-fields where the optical-signal reach ofunder several kilometers or even under several tens of meters isexpected.

A PIC typically uses optical waveguides to implement and/or interconnectvarious circuit components, such as optical switches, couplers, routers,splitters, polarization rotators, multiplexers/demultiplexers, filters,modulators, phase shifters, lasers, amplifiers, wavelength converters,optical-to-electrical (O/E) and electrical-to-optical (E/O) signalconverters, etc. A waveguide in a PIC is usually an on-chip solid lightconductor that guides light due to an index-of-refraction contrastbetween the waveguide's core and cladding. A PIC typically comprises aplanar substrate on which optical, electrical, and/or optoelectronicdevices are grown by an additive manufacturing process and/or into whichoptoelectronic devices are embedded by a subtractive manufacturingprocess, e.g., using a suitable sequence of photolithographic andchemical processing steps.

A “major plane” of an object, such as a die, a PIC, a substrate, or anelectronic integrated circuit (IC), is a plane parallel to asubstantially planar surface thereof that has the largest sizes, e.g.,length and width, among all exterior surfaces of the object. Thissubstantially planar surface may also be referred to as a main surface.The feature height variation along the main surface may typically bemuch smaller than length or width, or both length and width, of saidsurface. In such cases, such main surface may be referred to as asubstantially planar surface. The exterior surfaces of the object thathave one relatively large size, e.g., length, and one relatively smallsize, e.g., height, are typically referred to as the edges of theobject.

As used herein, the term “substrate” refers to a circuit or devicecarrier, a plate, a board, or a base designed and configured to providestructural support to various circuit elements and to support electricaland/or optical connections between different parts thereof to enableproper operation of electrical, optical, and/or optoelectroniccomponents located at, mounted on, or connected to those parts. Suchcomponents may include, e.g., different parts of an electronic IC or aPIC fabricated on the substrate or any combination of packaged ornon-packaged electronic integrated circuits, photonic integratedcircuits, and discrete (e.g., lumped) elements. Electrical connectionsbetween different parts of the substrate can be formed, e.g., usingpatterned electrically conducting (such as metal) layers located withinthe body or on the surface of the substrate and/or conventionalelectrical wiring. Optical connections between different optical and/oroptoelectronic components on the substrate can be formed using built-inoptical waveguides, optical couplers, and/or optical fibers or throughfree space, e.g., using discrete optical elements mounted on thesubstrate. In some embodiments, the substrate may have several distinctlevels, e.g., comprising a redistribution layer (RDL), an interposer, alaminated plate, and/or a printed circuit board.

In an example embodiment, a substrate can be implemented, e.g., using asilicon layer or base, a SOI wafer, or III-V semiconductor layer orbase. Typically, such a substrate has lateral dimensions (e.g., lengthand width) that are significantly larger than its thickness.

Integrated polarization-handling components of a PIC, such asconventional polarization splitters, combiners, and rotators, may beimplemented using on-chip waveguides having one or more slantedsidewalls, i.e., sidewalls that are not orthogonal to the correspondingmajor plane of the PIC or its substrate. Such slanted sidewalls can beformed, e.g., using anisotropic wet etching, which typically exhibitsrelatively large process variances and makes batch-to-batchreproducibility difficult to achieve. Since the guided optical modes canbe very sensitive to the slant angles and their profile(s) along thelongitudinal waveguide direction (i.e., the optical-signal propagationdirection), low chip yields may result, which may disadvantageously oreven prohibitively increase the cost of the corresponding PICs and ofthe assemblies and/or devices in which such PICs are used.

At least some of the above-indicated problems in the state of the artcan be addressed using PIC-design solutions that rely on laterallytilted waveguide cores, e.g., as disclosed herein. At least someembodiments can advantageously provide reliable polarization handling ina wavelength range between ca. 1.4 μm and 1.7 μm. Particularlybeneficial can be the concomitant ability to make the correspondingoptical waveguides using standard fabrication processes, e.g.,easy-to-control processes that are already in use in many semiconductorfoundries. Such processes may advantageously support relatively highchip yields and/or facilitate cost-effective fabrication of PDM-enabledPICs.

FIGS. 1A-1C show schematic views of different parts of an opticalwaveguide circuit 100 according to an embodiment. More specifically,FIG. 1B shows a schematic top view of circuit 100. FIG. 1A shows aschematic cross-sectional view of circuit 100 along the planarcross-section AA indicated in FIG. 1B. FIG. 1C shows a three-dimensionalperspective view of a waveguide core 114 used in circuit 100. TheXYZ-coordinate triad shown in each of FIGS. 1A-1C indicates the relativeorientation of the views shown in these figures.

Circuit 100 comprises an optical waveguide 110 formed and supported on amain surface 104 of a substrate 102. In an example embodiment, surface104 can be a substantially planar surface. Circuit 100 may be a part ofa larger PIC (not explicitly shown in FIGS. 1A-1C) formed and/orsupported on substrate 102. For illustration purposes, surface 104 isshown in FIGS. 1A-1B as being parallel to the XZ-coordinate plane.

Referring to FIG. 1A, optical waveguide 110 comprises waveguide core 114and a multi-part waveguide cladding, e.g., 112/116/118. Waveguide core114 is made of a material whose index of refraction (at the relevantwavelength) is higher than the refractive indices of the materials ofany of the cladding parts 112, 116, and 118. This characteristic enablesoptical waveguide 110 to guide light along the Z-coordinate direction,which may be referred to herein as the longitudinal waveguide direction.

Although example embodiments are described herein below in reference tostraight (i.e., linear) waveguide sections, such reference is notintended to be construed in a limiting sense. A person of ordinary skillin the pertinent art will readily understand that the disclosedinventive concepts also apply to curved optical waveguides. For example,for a curved optical waveguide, the orientation of the longitudinalwaveguide direction changes along the waveguide curve. A directionlocally orthogonal to the longitudinal waveguide direction and parallelto the substrate (e.g., parallel to surface 104) may be referred-to as alateral waveguide direction. The planar cross-section AA shown in FIG.1A is parallel to the XY-coordinate plane and is orthogonal to theZ-coordinate axis. In cross-section AA, the lateral waveguide directionis parallel to the X-coordinate axis. For a curved optical waveguide,the orientation of the lateral waveguide direction changes along thewaveguide curve in sync with the longitudinal waveguide direction.

The multi-part waveguide cladding of optical waveguide 110 comprises alower cladding 112, an upper cladding 116, and an encapsulating layer118. In some embodiments, encapsulating layer 118 may be absent. In suchembodiments, a ridge-waveguide structure 120 (which comprises lowercladding 112, waveguide core 114, and upper cladding 116) may besurrounded by air (or other gas or vacuum). In some embodiments, aportion of substrate 102 adjacent to ridge-waveguide structure 120 mayalso function as a part of the multi-part waveguide cladding (see, e.g.,FIGS. 2B-2C).

In an example embodiment, waveguide core 114 may comprise InGaAsP.Substrate 102, lower cladding 112, upper cladding 116, and encapsulatinglayer 118 may comprise variously doped, identically doped, and/orintrinsic InP.

Ridge-waveguide structure 120 has sidewalls 122 and 124. In an exampleembodiment, at least one of sidewalls 122 and 124 is substantially(e.g., within ±5 degrees or within ±2 degrees) orthogonal to surface 104of substrate 102. For illustration purposes, FIG. 1A shows an example inwhich each of sidewalls 122 and 124 is planar and orthogonal to surface104. Different planar portions of sidewall 122 correspond to lowercladding 112, waveguide core 114, and upper cladding 116, respectively.Similarly, different planar portions of sidewall 124 correspond to lowercladding 112, waveguide core 114, and upper cladding 116, respectively.

Although ridge-waveguide structure 120 is shown in FIG. 1B as having aconstant width (i.e., the size corresponding to the X-coordinate axis),embodiments in which the corresponding ridge-waveguide structure has avarying width are also contemplated.

In cross-section AA, waveguide core 114 has the cross-sectional shape ofa parallelogram, whose four sides are labeled a, b, c, and d,respectively. Sides a and c of the parallelogram are parallel to oneanother and on sidewalls 122 and 124, respectively. As such, sides a andc of the parallelogram are substantially orthogonal to surface 104 ofsubstrate 102. Sides b and d of the parallelogram are parallel to oneanother and are oriented at a tilt angle α with respect to surface 104.Side b is located at a larger offset distance with respect to (i.e., isfarther from) surface 104 than side d. As such, side b can be referredto as being a part of an upper surface of waveguide core 114, and side dcan be referred to as being a part of a lower surface of waveguide core114. In an example embodiment, the tilt angle α can be any nonzeroangle, e.g., in the range between 0 degrees and 60 degrees. The tiltangle α can be referred to as a lateral tilt angle because this anglecan be used to quantify the tilt of the upper and/or lower surfaces ofwaveguide core 114 with respect to surface 104.

In some sections of waveguide 110, the lateral tilt angle α of waveguidecore 114 may be constant along the longitudinal length of the section.In some other sections of waveguide 110, the lateral tilt angle α ofwaveguide core 114 may be changing in a continuous manner from a firsttilt-angle α₁ to a second tilt-angle α₂, where α₁≠α₂. In someembodiments, such change may be adiabatic with respect to the relevantproperties of the light guided by waveguide 110. In some embodiments, asection of waveguide 110 may include a juncture in which the value ofthe lateral tilt angle α changes in a substantially discontinuous (e.g.,step-like) manner from a first tilt angle α₁ to a second tilt angle α₂,where α₁≠α₂. In some embodiments, a section of waveguide 110 may includea step-like juncture in which an un-tilted section of the waveguide coreis connected to a tilted section of the waveguide core, i.e., whereinthe effective tilt angle changes from zero to a non-zero value α.

FIG. 1C pictorially shows an example three-dimensional shape ofwaveguide core 114 in a straight waveguide section characterized by aconstant lateral tilt angle α. In this case, waveguide core 114 has ashape of a polygonal prism, two bases of which, labeled 130 ₁ and 130 ₂,are congruent parallelograms. This polygonal prism also has fourrectangular lateral faces, labeled 132, 134, 136, and 138, respectively.

Base 130 ₁ has the previously mentioned four sides a, b, c, and d, whichare also shown in FIG. 1A. Lateral face 132 is a part of sidewall 122(also see FIG. 1A). Lateral face 136 is a part of sidewall 124 (also seeFIG. 1A). As such, each of lateral faces 132 and 136 is substantiallyorthogonal to surface 104 of substrate 102. Lateral faces 134 and 138represent the above-mentioned upper and lower surfaces, respectively, ofwaveguide core 114. Each of lateral faces 134 and 138 has a lateral tiltangle α with respect to the XZ-coordinate plane, as indicated in FIG.1C. As already mentioned above, surface 104 of substrate 102 is parallelto the XZ-coordinate plane.

Lateral faces 132 and 134 are connected to one another at an edge 133 ofthe polygonal prism. Lateral faces 134 and 136 are connected to oneanother at an edge 135 of the polygonal prism. Lateral faces 136 and 138are connected to one another at an edge 137 of the polygonal prism.Lateral faces 132 and 138 are connected to one another at an edge 139 ofthe polygonal prism.

In the example shown in FIG. 1C, lateral faces 132, 134, 136, and 138are flat, and edges 133, 135, 137, and 139 are straight. However, for awaveguide section having a continuously variable lateral tilt angle α,the upper and lower surfaces of waveguide core 114 are no longer flat,and the corresponding edges are not straight. For a curved waveguidesection, all of the lateral faces of the waveguide core are typicallynot flat, and the corresponding edges are not straight.

FIGS. 2A-2D show schematic cross-sectional views AA of severalalternative embodiments of ridge-waveguide structure 120 (also see FIG.1A). For added clarity, the optional encapsulating layer 118 is notexplicitly shown in any of FIGS. 2A-2D.

In the embodiment of FIG. 2A, ridge-waveguide structure 120 comprises alower cladding 212, waveguide core 114, and upper cladding 116. Lowercladding 212 differs from lower cladding 112 (FIG. 1A) in itscross-sectional shape. More specifically, lower cladding 212 has atriangular cross-sectional shape, whereas lower cladding 112 (FIG. 1A)has a trapezoid cross-sectional shape. In the shown example, thecross-section of lower cladding 212 has the shape of a right-angledtriangle, although other triangular shapes are also possible. Thetriangular cross-sectional shape of lower cladding 212 causes edge 137of waveguide core 114 to be in direct physical contact with (i.e., belocated directly on) surface 104 of substrate 102.

In the embodiment of FIG. 2B, ridge-waveguide structure 120 comprises awaveguide core 224 and upper cladding 116. Lower cladding 112 is notpresent. Waveguide core 224 differs from waveguide core 114 (FIGS. 1A,2A) in its cross-sectional shape. More specifically, the cross-sectionalshape of waveguide core 224 is a right-angled trapezoid. The bases ofthe trapezoid are on sidewalls 122 and 124, respectively. One leg of thetrapezoid has a zero tilt angle and is in direct physical contact with(i.e., is located on) surface 104 of substrate 102. The other (upper)leg of the trapezoid has a nonzero lateral tilt angle α₀. The embodimentof FIG. 2B can be viewed as a modification of the embodiment of FIG. 2A,in which lower cladding 212 (FIG. 2A) is made of the same material aswaveguide core 114, thereby extending the waveguide core down tosubstrate 102 and resulting in waveguide core 224.

In the embodiment of FIG. 2C, ridge-waveguide structure 120 comprises awaveguide core 234 and upper cladding 116. In this case, lower cladding112 is similarly not present, and the cross-sectional shape of waveguidecore 234 is a right-angled triangle. The lower side of the triangle isparallel to and is in direct physical contact with (i.e., is located on)surface 104 of substrate 102. The upper side of the triangle has anonzero lateral tilt angle α₀. The third side of the triangle is onsidewall 122. The triangular cross-sectional shape of waveguide core 234causes a lower corner edge 236 of upper cladding 116 to be in directphysical contact with (i.e., be located on) surface 104 of substrate102.

In the embodiment of FIG. 2D, ridge-waveguide structure 120 compriseslower cladding 112, a waveguide core 244, and upper cladding 116. Inthis case, the cross-sectional shape of waveguide core 244 is an obtusetrapezoid. The bases of the trapezoid are on sidewalls 122 and 124,respectively. One (i.e., the lower) leg of the trapezoid has a firstnonzero lateral tilt angle α₁. The other (upper) leg of the trapezoidhas a second nonzero lateral tilt angle α₂, where α₁<α₂.

FIG. 3A shows a simplified cross-sectional side view of a layeredstructure 300 that can be used in the process of manufacturing opticalwaveguide circuit 100 (FIGS. 1A-1C) according to an embodiment. TheXYZ-coordinate triad shown in FIG. 3A represents the same coordinatesystem as that shown in FIGS. 1A-1C. Some additional layers (such asetch-stop layers, buffer layers, and other technological layers known topersons of ordinary skill in the pertinent art) that may be present insome embodiments of layered structure 300 are not explicitly shown inFIG. 3A for clarity of depiction.

Layered structure 300 comprises substrate 102, a dielectric mask 302,and semiconductor layers 312, 314, and 316. In an example embodiment,substrate 102, layer 312, and layer 316 may be made of differently dopedInP. Layer 314 may be made of InGaAsP.

To arrive at optical waveguide circuit 100 starting from a baresubstrate 102, the following example manufacturing steps correspondingto layered structure 300 may be used:

-   -   (i) A planar dielectric mask 302 of appropriately chosen        thickness and in-plane shape is formed on surface 104 of        substrate 102, e.g., using lithography;    -   (ii) Selective Area Growth (SAG) epitaxy is then used to        sequentially deposit semiconductor layers 312, 314, and 316.        Under certain known conditions, SAG epitaxy may exhibit an        increased growth rate in the proximity of a dielectric mask,        such as mask 302. More specifically, the epitaxial growth rate        depends on the distance from the proximal edge of the dielectric        mask, such as an edge 304 of mask 302, and is typically higher        near the mask. The growth-rate gradient can be controlled by        adjusting the conditions under which the epitaxial growth is        carried out, e.g., as known to persons of ordinary skill in the        pertinent art. The thickness profiles of semiconductor layers        312, 314, and 316 shown in FIG. 3A represent just one example of        a layer-stack geometry that can be produced using such control.        For example, a layer stack having a different (from three)        number of semiconductor layers therein can similarly be        produced. In general, SAG epitaxy is known to be well        controllable and highly reproducible, which is beneficial, e.g.,        for achieving an acceptably high yield of the corresponding PICs        during manufacture;    -   (iii) A top surface 318 of semiconductors layer 316 is subjected        to chemical mechanical polishing (CMP) to produce a flat top        surface, e.g., parallel to surface 104 of substrate 102.        Patterning and etching is then used, as known in the pertinent        art, to remove the unwanted material outside the “corridor”        between dashed lines 320 and 322, down to surface 104. The        remaining portions of semiconductor layers 312, 314, and 316        within such corridor produce the ridge-waveguide structure 120        of FIG. 1A;    -   (iv) Optional encapsulating layer 118 may then be deposited over        the ridge-waveguide structure 120 obtained at step (iii) to        produce the optical waveguide circuit 100 of FIGS. 1A-1C.

From the example shown in FIG. 3A, a person of ordinary skill in the artwill understand that different lateral tilt angles α can be obtained bymoving the corridor 320-322 closer to edge 304 of mask 302 or fartherfrom said edge of the mask. A continuously changing lateral tilt angle αcan be obtained, e.g., by using a continuously curved mask 302 orientedsuch that different parts of the corridor 320-322 are located atdifferent respective distances from the curved edge 304 of the mask. Insuch cases, the in-plane shape of edge 304 determines the tilt-anglegradient along the longitudinal waveguide direction. Transitionscharacterized by abrupt (e.g., substantially discontinuous) changes ofthe lateral tilt angle α can be obtained, e.g., by using a dielectricmask 302 whose edge 304 has an appropriately oriented zigzag shape inthe plane of surface 104.

FIGS. 3B-3C schematically show two example in-plane layouts of mask 302that can be used in some embodiments of the example manufacturingprocess described above in reference to FIG. 3A. More specifically, thecross-sectional view of FIG. 3A corresponds to the cross-sectional planeFF shown in each of FIGS. 3B and 3C. The XYZ coordinate triads shown inFIGS. 3B and 3C represent the same coordinate system as that shown inFIG. 3A.

In the example of FIG. 3B, edge 304 of mask 302 comprises three linearsegments, labeled 304 ₁, 304 ₂, and 304 ₃, respectively. Segments 304 ₁and 304 ₃ are parallel to the boundaries 320 and 322 of the nascentridge-waveguide structure 120. Segment 304 ₁ is closer to the boundary322 than segment 304 ₃. Segment 304 ₂ is not orthogonal to the boundary322 and connects the corresponding ends of segments 304 ₁ and 304 ₃ asindicated in FIG. 3B.

With this mask layout, the ridge-waveguide structure 120, which isformed after the above-described manufacturing step (iii), has threesections, labeled 120 ₁, 120 ₂, and 120 ₃, respectively. Within section120 ₁, the lateral tilt angle α is substantially constant and has afirst value, e.g., α=α₁. Within section 120 ₃, the lateral tilt angle αis also substantially constant and has a second value, e.g., α=α₃, whereα₃<α₁. Along section 120 ₂, the lateral tilt angle α changes graduallyfrom the first value to the second value.

In the example of FIG. 3C, segment 304 ₂ is orthogonal to the boundary322. With this particular mask layout, section 120 ₂ is not present inthe corresponding ridge-waveguide structure 120. Rather, in a narrowvicinity of the juncture 306 between sections 120 ₁ and 120 ₃, the valueof the lateral tilt angle α changes relatively sharply from the firsttilt angle value α₁ to the second tilt angle value α₃.

FIG. 4 shows a schematic cross-sectional view AA of ridge-waveguidestructure 120 (FIG. 1A) according to yet another embodiment. In theembodiment of FIG. 4, ridge-waveguide structure 120 comprises lowercladding 112, a waveguide core 414, and upper cladding 116. Waveguidecore 414 functionally differs from the above-described waveguide cores,e.g., 114 (FIG. 1A), in that waveguide core 414 can be configured tofunction as an active optical element, whereas the other above-describedwaveguide cores are typically configured to function as passive opticalelement.

As used herein, the term “passive optical component or element” refersto an optical component or element that does not require externalelectrical power to operate. Most passive optical components/elements donot generate new light, and simply handle in a predetermined manner theexternally generated light applied thereto. Representative examples ofpassive optical components and elements include, but are not limited tooptical connectors and fixed signal splitters, attenuators, isolators,couplers, filters, etc.

In contrast, the term “active optical component or element” refers to anoptical component or element designed to be controllable and/or operableusing an external electrical (e.g., control, power, or pump) signal.Some active optical components and elements can generate new light. Someother active optical components and elements can variably andcontrollably attenuate light in response to an external electricalsignal. Representative examples of active optical components andelements include, but are not limited to semiconductor lasers,semiconductor optical amplifiers, electro-absorption modulators, etc.

Waveguide core 414 comprises a stack of relatively thin, laterallytilted semiconductor layers. In an example embodiment, both the uppersurface and the lower surface of waveguide core 414 may have the samelateral tilt angle α, e.g., as indicated in FIG. 4. The stack of layerscomprises a multiple-quantum-well (MQW) structure formed by alternatingrelatively thin layers 404 and 406 made of different respectivesemiconductor materials. In some embodiments, the semiconductormaterials of layers 404 and 406 may be intrinsic semiconductors. Lowercladding 112 and upper cladding 116 may be made of n-doped and p-dopedsemiconductor materials, respectively.

In an example embodiment, the following semiconductor materials can beused to implement layers 404-406: (i) In(x)Ga(1-x)As for layers 404; and(ii) In(x)Al(1-x)As for layers 406. In alternative embodiments, othersemiconductor materials and/or dopants can similarly be used.

Lower cladding 112 and upper cladding 116 and the alternating layers404/406 therebetween form a p-i-n diode (also sometimes referred to as a“PIN diode”) that can be electrically biased using electrodes (notexplicitly shown in FIG. 4) connected to lower cladding 112 and uppercladding 116. In operation, such electrodes may be electricallyconnected to apply to the PIN diode an appropriate dc bias or acombination of the dc bias and a radio-frequency (RF) signal. Indifferent electrical configurations of the PIN diode, the dc bias can bea reverse bias or a forward bias.

As used herein, the term “reverse bias” refers to an electricalconfiguration of a semiconductor-junction diode in which the n-typematerial is at a high electrical potential, and the p-type material isat a low electrical potential. The reverse bias typically causes thedepletion layer to grow wider due to a lack of electrons and/or holes,which presents a high impedance path across the junction andsubstantially prevents a current flow therethrough. However, a verysmall reverse leakage current can still flow through the junction in thereverse-bias configuration.

Similarly, the term “forward bias” refers to an electrical configurationof a semiconductor-junction diode in which the n-type material is at alow potential, and the p-type material is at a high potential. If theforward bias is greater than the intrinsic voltage drop V_(pn) acrossthe corresponding p-i-n junction, then the corresponding potentialbarrier can be overcome by the electrical carriers, and a relativelylarge forward current can flow through the junction. For example, forsilicon-based diodes the value of V_(pn) is approximately 0.7 V. Forgermanium-based diodes, the value of V_(pn) is approximately 0.3 V, etc.

In some embodiments, waveguide core 414 may also comprise optionalconfinement heterostructures 410 at the upper and lower sides thereof,e.g., as indicated in FIG. 4. Such confinement heterostructures 410 maybe used to better confine light in the above-described MQW structure,e.g., to improve efficiency thereof.

In various alternative embodiments of ridge-waveguide structure 120, MQWstructures based on other, e.g., more complex layer stacks and/or dopingprofiles, can be realized as well. For example, n-i-n or n-i-p-nsemiconductor structures may be implemented in some of such alternativeembodiments.

FIGS. 5A-5D show schematic views of an optical waveguide circuit 500according to another embodiment. More specifically, FIG. 5A shows aschematic top view of circuit 500. FIG. 5B shows a schematiccross-sectional view of circuit 500 along the planar cross-section BB orEE indicated in FIG. 5A. FIG. 5C shows a schematic cross-sectional viewof circuit 500 along the planar cross-section CC indicated in FIG. 5A.FIG. 5D shows a schematic cross-sectional view of circuit 500 along theplanar cross-section DD indicated in FIG. 5A. The XYZ-coordinate triadshown in FIGS. 5A and 5B indicates the relative orientation of the viewsshown in these two figures. The views shown in FIGS. 5C and 5D have thesame orientation as the view of FIG. 5B.

Circuit 500 comprises a ridge-waveguide structure 520 formed andsupported on main surface 104 of substrate 102. Ridge-waveguidestructure 520 is generally similar to ridge-waveguide structure 120 buthas four different waveguide sections labeled 520 ₁, 520 ₂, 520 ₃, and520 ₄, respectively. In operation, an optical input signal 502 appliedto an end 518 of waveguide section 520 ₁ sequentially traverseswaveguide sections 520 ₁, 520 ₂, 520 ₃, and 520 ₄ and comes out an end538 of the latter waveguide section as an optical output signal 504.Optical signals 502 and 504 may differ from one another in theirpolarization state, e.g., as explained in more detail below.

Referring to FIG. 5B, waveguide section 520 ₁ comprises a lower cladding512, a waveguide core 514, and an upper cladding 516. Waveguide core 514may be referred to as being untilted or flat. More specifically, each oflower cladding 512, waveguide core 514, and upper cladding 516 has arespective rectangular cross-sectional shape. As a result, both a lowersurface 513 and an upper surface 515 of waveguide core 514 are planarand parallel to surface 104 of substrate 102. This orientation ofsurfaces 513 and 515 can also be described as corresponding to a zerolateral tilt angle α, i.e., α=0. Waveguide core 514 is untilted alongthe entire longitudinal length of waveguide section 520 ₁.

Waveguide section 520 ₄ has the same structure as waveguide section 520₁. As such, FIG. 5B represents both the cross-section BB of waveguidesection 520 ₁ and the cross-section EE of waveguide section 520 ₄.

Referring to FIG. 5C, waveguide section 520 ₂ comprises a lower cladding522, a waveguide core 524, and an upper cladding 526. The lateral tiltangle α of waveguide core 514 changes in a continuous manner along thelongitudinal direction of waveguide section 520 ₂ from a zero value(i.e., α=0) at a junction 542 between waveguide sections 520 ₁ and 520 ₂to a nonzero tilt angle α₂ at a junction 544 between waveguide sections520 ₂ and 520 ₃. As a result, the cross-sectional shape of waveguidecore 524 changes from being a rectangle at junction 542 to being aparallelogram at junction 544. In the cross-sectional plane CC, which islocated in a middle longitudinal part of waveguide section 520 ₂,waveguide core 524 has a nonzero lateral tilt angle α₁, where 0<α₁<α₂.

Referring to FIG. 5D, waveguide section 520 ₃ comprises a lower cladding532, a waveguide core 534, and an upper cladding 536. The lateral tiltangle α of waveguide core 534 is constant along the entire longitudinallength of waveguide section 520 ₃. The cross-sectional shape ofwaveguide core 534 is a fixed parallelogram, which is shown in FIG. 5D.At a junction 546 between waveguide sections 520 ₃ and 520 ₄, thecross-sectional shape of the waveguide core changes in an abrupt mannerfrom being a parallelogram (as shown in FIG. 5D) to being a rectangle(as shown in FIG. 5B).

In an example embodiment, circuit 500 can function as a polarizationsplitter-rotator. For example, let us assume that optical input signal502 has a TE polarization. Herein, the TE polarization of light incircuit 500 has the electric field oriented substantially along theY-coordinate axis. In contrast, the TM polarization of light in circuit500 has the electric field oriented substantially along the X-coordinateaxis.

Optical input signal 502 may couple into one or more TE-polarizedeigenmodes of waveguide section 520 ₁. Waveguide section 520 ₁ thenguides the resulting coupled TE-polarized optical signal to junction 542and applies said signal thereat to waveguide section 520 ₂. The gradualchange of the lateral tilt angle α of waveguide core 524 along thelongitudinal waveguide direction in waveguide section 520 ₂ causes thecorresponding optical signal at junction 544 to be in a hybridpolarization mode. Herein, the term “hybrid polarization mode” refers toa guided mode in which light of both TE and TM polarizations is present.

In an example embodiment, waveguide section 520 ₃ may support two ormore hybrid polarization modes. Depending on the implementationspecifics of waveguide sections 520 ₂ and 520 ₃, a single one of suchhybrid polarization modes or more than one of such hybrid polarizationmodes may be populated at junction 546. From junction 546, the outputhybrid polarization mode(s) of waveguide section 520 ₃ may be coupledinto waveguide section 520 ₄ as a linear combination of TE and TMeigenmodes therein. Waveguide section 520 ₄ then outputs said linearcombination of TE and TM eigenmodes at end 538 thereof as optical outputsignal 504, which may be further directed to downstream circuits of thecorresponding PIC.

The power ratio of TE- and TM-polarized light in optical output signal504 typically depends on the implementation specifics of circuit 500.For example, various parameters of waveguide sections 520 ₁, 520 ₂, 520₃, and 520 ₄ may be selected such that a 50/50 power ratio between TE-and TM-polarized light in optical output signal 504 is produced inresponse to a TE-polarized optical input signal 502. Such an embodimentof circuit 500 thus functions as a 3-dB polarization splitter-rotator. Aperson of ordinary skill in the art will understand that otherembodiments of circuit 500 may be constructed to produce other TE/TMpower ratios and/or to operate on differently polarized optical inputsignals 502.

In some embodiments of circuit 500, waveguide section 520 ₃ may beabsent.

According to an example embodiment disclosed above, e.g., in the summarysection and/or in reference to any one or any combination of some or allof FIGS. 1A-5D, provided is an apparatus, comprising a photonicintegrated circuit that comprises: a substrate (e.g., 102, FIG. 1A)having a substantially planar main surface (e.g., 104, FIG. 1A); and anoptical waveguide (e.g., 110, FIG. 1A) attached to the substrateadjacent and along said main surface, the optical waveguide including awaveguide core (e.g., 114, FIG. 1A); and wherein a first section of thewaveguide core has a first edge (e.g., 133, FIG. 1C) whereat first andsecond surfaces (e.g., 132, 134, FIG. 1C) of the waveguide core areconnected to one another, the first surface being substantiallyorthogonal to said main surface, the second surface having a nonzerolateral tilt angle (e.g., α, FIG. 1C) with respect to said main surface.

In some embodiments of the above apparatus, the nonzero lateral tiltangle changes continuously along a longitudinal waveguide direction ofthe first section between a first nonzero tilt-angle value (e.g., α₁,FIG. 5C) and a different second nonzero tilt-angle value (e.g., α₂, FIG.5D).

In some embodiments of any of the above apparatus, the nonzero lateraltilt angle is greater than 5 degrees but smaller than 60 degrees.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge (e.g., 139, FIG. 1C) whereat thefirst surface and a third surface (e.g., 138, FIG. 1C) of the waveguidecore are connected to one another, the third surface having the nonzerolateral tilt angle (e.g., α, FIG. 1C) with respect to said main surface.

In some embodiments of any of the above apparatus, the second edge iscloser to said main surface than the first edge.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge whereat the first surface and athird surface (e.g., lower surface of 244, FIG. 2D) of the waveguidecore are connected to one another, the third surface having a smallernonzero lateral tilt angle with respect to said main surface than thesecond surface (e.g., α₁<α₂, FIG. 2D).

In some embodiments of any of the above apparatus, the first section ofthe waveguide core has a second edge whereat the first surface and athird surface (e.g., lower surface of 224, FIG. 2B) of the waveguidecore are connected to one another, the third surface being parallel tosaid main surface.

In some embodiments of any of the above apparatus, the first section ofthe waveguide core comprises a semiconductor diode (e.g., 414, FIG. 4).

In some embodiments of any of the above apparatus, the first section ofthe waveguide core comprises a multiple-quantum-well structure having aplurality of semiconductor layers (e.g., 404, 406, FIG. 4), at leastsome of said semiconductor layers being parallel to the second surface.

In some embodiments of any of the above apparatus, a second section ofthe waveguide core is untilted with respect to said main surface, thefirst and second sections of the waveguide core being end-connected toone another (e.g., 520 ₃/520 ₄ at 546, FIG. 5A).

In some embodiments of any of the above apparatus, the photonicintegrated circuit comprises a polarization splitter-rotator (e.g., 500,FIG. 5A) that includes the first and second sections of the waveguidecore.

In some embodiments of any of the above apparatus, a second section ofthe waveguide core has a different nonzero lateral tilt angle, the firstand second sections of the waveguide core being end-connected to oneanother (e.g., 520 ₂/520 ₃ at 544, FIG. 5A).

In some embodiments of any of the above apparatus, a third section ofthe waveguide core is untilted with respect to said main surface, thesecond and third sections of the waveguide core being end-connected toone another (e.g., 520 ₃/520 ₄ at 546, FIG. 5A).

In some embodiments of any of the above apparatus, a third section ofthe waveguide core is untilted with respect to said main surface, thefirst and third sections of the waveguide core being end-connected toone another (e.g., 520 ₂/520 ₁ at 542, FIG. 5A).

In some embodiments of any of the above apparatus, the photonicintegrated circuit comprises a polarization splitter-rotator (e.g., 500,FIG. 5A) that includes the first, second, and third sections of thewaveguide core.

In some embodiments of any of the above apparatus, the optical waveguidecomprises a ridge-waveguide structure (e.g., 120, FIG. 1A) attached tothe substrate adjacent and along said main surface, the ridge-waveguidestructure having first and second sidewalls (e.g., 122, 124, FIG. 1A)substantially orthogonal to said main surface, the first sidewallincluding the first surface.

In some embodiments of any of the above apparatus, the ridge-waveguidestructure includes at least a portion of a waveguide cladding, saidportion (e.g., part of 116, FIG. 1A) being farther from said mainsurface than the first section of the waveguide core.

While this disclosure includes references to illustrative embodiments,this specification is not intended to be construed in a limiting sense.Various modifications of the described embodiments, as well as otherembodiments within the scope of the disclosure, which are apparent topersons skilled in the art to which the disclosure pertains are deemedto lie within the principle and scope of the disclosure, e.g., asexpressed in the following claims.

Unless explicitly stated otherwise, each numerical value and rangeshould be interpreted as being approximate as if the word “about” or“approximately” preceded the value or range.

It will be further understood that various changes in the details,materials, and arrangements of the parts which have been described andillustrated in order to explain the nature of this disclosure may bemade by those skilled in the art without departing from the scope of thedisclosure, e.g., as expressed in the following claims.

The use of figure numbers and/or figure reference labels in the claimsis intended to identify one or more possible embodiments of the claimedsubject matter in order to facilitate the interpretation of the claims.Such use is not to be construed as necessarily limiting the scope ofthose claims to the embodiments shown in the corresponding figures.

Although the elements in the following method claims, if any, arerecited in a particular sequence with corresponding labeling, unless theclaim recitations otherwise imply a particular sequence for implementingsome or all of those elements, those elements are not necessarilyintended to be limited to being implemented in that particular sequence.

Reference herein to “one embodiment” or “an embodiment” means that aparticular feature, structure, or characteristic described in connectionwith the embodiment can be included in at least one embodiment of thedisclosure. The appearances of the phrase “in one embodiment” in variousplaces in the specification are not necessarily all referring to thesame embodiment, nor are separate or alternative embodiments necessarilymutually exclusive of other embodiments. The same applies to the term“implementation.”

Unless otherwise specified herein, the use of the ordinal adjectives“first,” “second,” “third,” etc., to refer to an object of a pluralityof like objects merely indicates that different instances of such likeobjects are being referred to, and is not intended to imply that thelike objects so referred-to have to be in a corresponding order orsequence, either temporally, spatially, in ranking, or in any othermanner.

Unless otherwise specified herein, in addition to its plain meaning, theconjunction “if” may also or alternatively be construed to mean “when”or “upon” or “in response to determining” or “in response to detecting,”which construal may depend on the corresponding specific context. Forexample, the phrase “if it is determined” or “if [a stated condition] isdetected” may be construed to mean “upon determining” or “in response todetermining” or “upon detecting [the stated condition or event]” or “inresponse to detecting [the stated condition or event].”

Throughout the detailed description, the drawings, which are not toscale, are illustrative only and are used in order to explain, ratherthan limit the disclosure. The use of terms such as height, length,width, top, bottom, is strictly to facilitate the description of theembodiments and is not intended to limit the embodiments to a specificorientation. For example, height does not imply only a vertical riselimitation, but is used to identify one of the three dimensions of athree dimensional structure as shown in the figures. Such “height” wouldbe vertical where the layers are horizontal but would be horizontalwhere the layers are vertical, and so on. Similarly, while all figuresshow the substrates as being horizontal, such orientation is fordescriptive purpose only and not to be construed as a limitation.

Also for purposes of this description, the terms “couple,” “coupling,”“coupled,” “connect,” “connecting,” or “connected” refer to any mannerknown in the art or later developed in which energy is allowed to betransferred between two or more elements, and the interposition of oneor more additional elements is contemplated, although not required.Conversely, the terms “directly coupled,” “directly connected,” etc.,imply the absence of such additional elements. The same type ofdistinction applies to the use of terms “attached” and “directlyattached,” as applied to a description of a physical structure. Forexample, a relatively thin layer of adhesive or other suitable bindercan be used to implement such “direct attachment” of the twocorresponding components in such physical structure.

The described embodiments are to be considered in all respects as onlyillustrative and not restrictive. In particular, the scope of thedisclosure is indicated by the appended claims rather than by thedescription and figures herein. All changes that come within the meaningand range of equivalency of the claims are to be embraced within theirscope.

The description and drawings merely illustrate the principles of thedisclosure. It will thus be appreciated that those of ordinary skill inthe art will be able to devise various arrangements that, although notexplicitly described or shown herein, embody the principles of thedisclosure and are included within its spirit and scope. Furthermore,all examples recited herein are principally intended expressly to beonly for pedagogical purposes to aid the reader in understanding theprinciples of the disclosure and the concepts contributed by theinventor(s) to furthering the art, and are to be construed as beingwithout limitation to such specifically recited examples and conditions.Moreover, all statements herein reciting principles, aspects, andembodiments of the disclosure, as well as specific examples thereof, areintended to encompass equivalents thereof.

“SUMMARY OF SOME SPECIFIC EMBODIMENTS” in this specification is intendedto introduce some example embodiments, with additional embodiments beingdescribed in “DETAILED DESCRIPTION” and/or in reference to one or moredrawings. “SUMMARY OF SOME SPECIFIC EMBODIMENTS” is not intended toidentify essential elements or features of the claimed subject matter,nor is it intended to limit the scope of the claimed subject matter.

What is claimed is:
 1. An apparatus, comprising a photonic integratedcircuit that comprises: a substrate having a substantially planar mainsurface; and an optical waveguide attached to the substrate adjacent andalong said main surface, the optical waveguide including a waveguidecore; and wherein a first section of the waveguide core has a first edgewhereat first and second surfaces of the waveguide core are connected toone another, the first surface being substantially orthogonal to saidmain surface, the second surface having a nonzero lateral tilt anglewith respect to said main surface.
 2. The apparatus of claim 1, whereinthe nonzero lateral tilt angle changes continuously along a longitudinalwaveguide direction of the first section between a first nonzerotilt-angle value and a different second nonzero tilt-angle value.
 3. Theapparatus of claim 1, wherein the nonzero lateral tilt angle is greaterthan 5 degrees but smaller than 60 degrees.
 4. The apparatus of claim 1,wherein the first section of the waveguide core has a second edgewhereat the first surface and a third surface of the waveguide core areconnected to one another, the third surface having the nonzero lateraltilt angle with respect to said main surface.
 5. The apparatus of claim1, wherein the first section of the waveguide core has a second edgewhereat the first surface and a third surface of the waveguide core areconnected to one another, the third surface having a smaller nonzerolateral tilt angle with respect to said main surface than the secondsurface.
 6. The apparatus of claim 1, wherein the first section of thewaveguide core has a second edge whereat the first surface and a thirdsurface of the waveguide core are connected to one another, the thirdsurface being parallel to said main surface.
 7. The apparatus of claim1, wherein the first section of the waveguide core comprises asemiconductor diode.
 8. The apparatus of claim 1, wherein the firstsection of the waveguide core comprises a multiple-quantum-wellstructure having a plurality of semiconductor layers, at least some ofsaid semiconductor layers being parallel to the second surface.
 9. Theapparatus of claim 1, wherein a second section of the waveguide core isuntilted with respect to said main surface, the first and secondsections of the waveguide core being end-connected to one another. 10.The apparatus of claim 9, wherein the photonic integrated circuitcomprises a polarization splitter-rotator that includes the first andsecond sections of the waveguide core.
 11. The apparatus of claim 1,wherein a second section of the waveguide core has a different nonzerolateral tilt angle, the first and second sections of the waveguide corebeing end-connected to one another.
 12. The apparatus of claim 11,wherein a third section of the waveguide core is untilted with respectto said main surface, the second and third sections of the waveguidecore being end-connected to one another.
 13. The apparatus of claim 11,wherein a third section of the waveguide core is untilted with respectto said main surface, the first and third sections of the waveguide corebeing end-connected to one another.
 14. The apparatus of claim 13,wherein the photonic integrated circuit comprises a polarizationsplitter-rotator that includes the first, second, and third sections ofthe waveguide core.
 15. The apparatus of claim 1, wherein the opticalwaveguide comprises a ridge-waveguide structure attached to thesubstrate adjacent and along said main surface, the ridge-waveguidestructure having first and second sidewalls substantially orthogonal tosaid main surface, the first sidewall including the first surface; andwherein the ridge-waveguide structure includes at least a portion of awaveguide cladding, said portion being farther from said main surfacethan the first section of the waveguide core.